1. Field of the Invention
The present invention relates generally to a method for synchronization in encrypted communications using a shared key. More particularly, the invention relates to a technique for synchronization of a running key used for encryption and decryption between a transmitting node and a receiving node when communicating data encrypted with a shared key using a multi-valued signal for quantum cryptographic transmission in optical communications, and relates to a method for encrypted communications using the technique, a system for the technique, and a receiving device used in the system.
2. Description of the Related Art
In communication technologies, a communication signal is generally encrypted. For the encryption, there are two known encryption schemes: a software encryption (mathematical encryption) scheme using a mathematical analysis technique; and a physical encryption scheme using physical effects of a signaling system.
The software encryption scheme provides a lot of combinations of codes so as to ensure difficulty in deciphering them. However, data encrypted by using the software encryption scheme can be in principle deciphered by matching using all possible combinations of codes (brute force key search attacks). In contrast, the purpose of the physical encryption scheme is to prevent data from being deciphered in principle. A quantum encryption scheme, which has been under development in recent years, is of the type of the latter.
As disclosed in Document 1 (C. H. Bennett and G. Brassard, “Quantum cryptography,” in Proc. IEEE, Int. Conf, on Computers system, and signal processing, p. 175, (1984)), a secret key distribution protocol (BB-84) proposed by C. H. Bennett and G. Brassard in 1984 is known as a quantum encryption scheme. Document 1 has proposed that a distribution of a large amount of keys required for the one time pad scheme be achieved using a technique for quantum communications.
In addition, another quantum encryption scheme, a shared key quantum encryption scheme has been under development, in which quantum fluctuations (quantum shot noise) of light are spread by modulation so that an eavesdropper cannot properly receive an optical signal and that data cannot be deciphered even by infinite computational power. The shared key quantum encryption scheme is designed so that a binary optical signal carrying data to be transmitted in a binary code is regarded as one set (which is called a basis), and that an M number of bases is prepared, and that a basis used to transmit data is randomly determined among the bases based on a pseudo-random number generated in accordance with an encryption key. Practically, a multi-valued optical signal is designed such that an inter-signal distance is small enough to prevent data identification due to quantum fluctuations. Thus, with the shared key quantum encryption scheme, the eavesdropper cannot decipher data information from a received signal.
Optical modulation/demodulation devices used in the legitimate transmitting and receiving nodes operate to communicate data while changing the M number of the binary bases in accordance with common pseudo-random numbers. Thus, the legitimate receiving node can read data by detecting a binary signal having a large inter-signal distance. As a result, an error due to quantum fluctuations can be ignored, whereby the shared key encryption scheme makes it possible to provide appropriate communications.
The encryption scheme based on the abovementioned principle is called Yuen quantum cryptography, which is based on Yuen-2000 cryptographic communication protocol (or ALPHA-ETA protocol in US, abbreviated to Y-00 Protocol). Northwestern University where P. Kumar, H. Yuen, et al. are enrolled has disclosed an optical phase modulation technique, which is a communication scheme that allows for the Y-00 Protocol, in Document 2 (G. A. Barbosa, E. Corndorf, P. Kumar, H. P. Yuen, “Secure communication using mesoscopic coherent state,” Phys. Rev. Lett. Vol-90, 227901, (2003)). In addition, a group of Tamagawa University has disclosed an optical intensity modulation technique in Document 3 (O. Hirota, K. Kato, M. Sohma, T. Usuda, K. Harasawa, “Quantum stream cipher based on optical communication” SPIE Proc. on Quantum Communications and Quantum Imaging vol-5551, (2004)).
It should be noted that the Y-00 Protocol can provide higher cryptographic strength in, e.g., radio communications and electrical communications in which quantum fluctuations are not used (called classical Y-00 Protocol), as compared with typical stream encryption schemes.